Polarity reverser



Sept. 25, 1951 GE. G. GRAHAM POLARITY REVERSER Filed oct. 28': 1947 FIG, l;

- GUlESCE/ VT ANODE CURRENT FIG.;

il A INVENTR;

' lTGEoRGE E G. GRAHAM Patented Sept. 25, v.1951

UNITED -S .'l' iQiFFIC-E yPOLA-RITY REVERSER George lE56Graham, StreathamfLondon, England, assignor -to Cinemalelevision.Limited London, England, a corporation..ofv England ApplicationOctoberZS, 1947,?Serial. No.' 782,551 In Great Britain May1151947 .4 claims. (c1. 25o- 227) This invention relates to Vimprovements in methods of andapparatus for controlling electri- .cal signals and is especially concernedv with new 'and improved means for selectively controlling 'the polarity of such signals.

For certain purposes it is desirable to be able to control'the polarity of electrical-signals,v that to lbe able to arrange that signalsv derived from "one part of a circuit shall be applied `to another .part of the circuit in either positive or'negative fpolarill, the control of the polarity-being effected .bymeans of a substantially inertia-free device. Means foreeiecting such control have hitherto .-,been' relatively complex, consisting in general of two signal channels renderedl alternatively operative, one of^said channels being varranged to rev-verse the polarity. ofsignals applied thereto, lwhile the other channel passes signals Without changing their polarity. Arrangements-ofvthis .type Tsuffer from the disadvantages of complexity and ...signal vdistortion due to the imperfectionsof the .'polarity inverter and the 'excessive Vstray capacities introduced intothe' circuit.

Thepresent invention seeksto overcome the aforesaid disadvantages by making .use of a valve .of the type comprisinga cathode, an anode,` from vwhich vsignals are 'to be derived, a first control .1 electrode adapted! to control the total emission yfrom said cathode and at leastanother control :electrodeadapted to produce a variation of current withinsaid valveV such that the slope ofthe .characteristic of the anode current versus'rst vcontrol electrode potential is thereby reversed. ..=`Accordingto the present inventionthere provided a method of selectivelyl controlling: the polarity of electrical signalsgwhichmethod con- -sists in applying said signals to the firstfcontrol electrode of a valve of the type defined,r deriving controlled signals across an impedance nconnected `m the anode clrcult of Sald Valve and Varymg the 40 :resented fby-'curve;fB,Y theroutputsignal will'xbe of ipotential of asecond controlelectro'denof said Lvalve so as to cause the derived signals .to have the desired polarity.

Equally according to the present rinvention `ithere' is provided a circuit -for selectivelyl controlling the polarity :of electrical. signals comprising a valve of the type dened to .arstrcontrol electrode of which signals are applied and fromthe anode of which controlled signals are derived and means for varying the potential of a second control-#electrode of saidy valve so as to `cause the `derivedsignals to have :the desired polarity.

In order that-the present-invention may be more fully described, reference isv now made to theaccompanying drawings of which y `Figurel vis a diagramillustrating certairr voltage-current relationships within Va valve' f the Ltype'dened,

Figure 2 is a circuit diagram of one embodi- .I2 ment of .apparatus according to `the invention, and K FigureA 3-is aseries of waveforms. illustrative .fof theloperatio'n of thecircuitshownin FigureQZ. Y

Referringnow to..1ig.ureV 1, the curve marked A illustrates the shape of the characteristic Lof 4anode current Iversus signal-gridpotential (if-fone ytype of. lscreen grid valve when .the .potential on `th'efanode of theivalve is. approximately 90volts .and that 'on the. screen grid -is v approximately. 130 volts. Y.It will be yseen that-the. slop of curveA isfnegativeVthat is, the .anode rcurrent becomes progressively smaller as thesignal-gridl potential becomes progressivelymorefpositive vThis phenomenon-.is Well known, Yandfisy due toy theiact .that each electro-n. incident. upon.. the anodeca'us'es .the ejection from theanodefo-f oneor; more other electrons, whichare drawn towardsand collected .by the screen grid. The4 number of l,electrc'ms .co1-

lected by the screen vgrid Ais vdetermine'dlby the Yhighscreen potential relative to the anode. ...Thus .the total-.espace current-flowing Ato vthe-anodefis fof negative'polaritm that is, more electrons Aleave the anode than arrive at it. This Vlis.k1iovvn'las fsecondary emissio-n. A A y n 1The curvefBgcn the'other hand, illustrates,- the :anodea currentversus.- signal-grid potential. charl acteristic of :the same *.-type -f of valve when :the fanode potential isstill approximately AHvoltsand the screenggrid potential is reducedf to .substantially' thersameva-lue. +It-will be seen that in this case Lthe slope trof the ycurve is jpositive, that is, changes ini-'grid` potential; 'im-a positive direction :resu1t in 1 increases iin l.anode current. .'.ILLthereforepan input signalbeappliedy to the fsignal-'gridiofa'valve inthe condition represented by curve A, the outputsignal Valirpearing-on,- its .'fanodezwi-ll have-,the same polarity asthe applied signaLi-While if? .the valve .be :in the-conditionl rep- Xreversedpolarity. SAS"thexdiierencefbetween con.- fzditions .A fandiB lis thatzof .a :difference zinrthe f screengridpotential, itzwillA-be seen that` the `valve ."maye'rapidlycbe changed; from :one Iconditionzto vthe'rotherzby applyinafsuitable `change impotenftialtoithe screencgri'd. I

:Iigurei-:z illustrates .ani emboldimentfof Vfa -ciracuitaccordingLto the;present:'invention. A screen grid valve. I i'hasc-tsffcathode .connected to.: ground lthroughzaibiasingi. resistance I3 and its anode connected=zto a sourcexof suitable energizing .potentialiB-ithrough a vload resistance. 2. Outputsignals which appear-fat the. anode of .the valve I 'are available to iai utilization circuit over blocking condenser V3 and across output terminals14.

"-Input signals -areapplied tothe circuit-lacross terminals*ffromfwhichthey are fedto the-sig- 3 turned toground through grid resistances 'I and 8 in series.

The screen grid'of the valve is connected to a source of energizing potential, for example to the source, B| through an appropriate dropping resistance II having a value suitable for reducing the B+ voltage to the normal screen grid working voltage. v

Control voltages are applied across terminals 9 from which they are fed to the screen grid of valve I over blocking condenser Ill. It is customary in most applications of screen grid tubes to ground the screen grid as to varying voltages by connecting it to ground over a condenser thus by-passing the screen power supply including the direct current dropping resistor, with a low impedance element, i. e. an element having a low value of capacitative reactance. In the present application the screen grid may be suitably stabilized by employing a source of control voltages which has a low internal or output impedance.

For reasons stated below it is desirable also to apply the control voltages to the signal grid and to apply them there in reduced amplitude. For this purpose the control signals are also applied across a voltage divider comprising resistors I2 and 8 and the portion thereof which appears across resistor 8 is applied to the signal grid over the resistor Since Figure 2 is diagrammatic, well known circuit elements, such as the cathode heater and its supply circuit, are omitted. It is obvious, however, that in constructing actual embodiments of this invention these elements must be included and may be arranged in accordance with general practice,

for example, for most embodiments it will be necessary to use a blocking condenser between the lower end of resistances il and the upper end of resistance I2.

The operation of the circuit will now be described with reference also to Figure 3, which illustrates the voltage waveforms appearing at different parts of the circuit of Figure 2 under certain conditions. control voltages applied across terminals 9 and thence to the screen grid of the valve I, waveform b exemplifies the wave form of input signals applied to terminals and thence to the signal grid of the valve, and waveform c is that of the corresponding output voltages appearing at terminals It under the operating conditions further discussed below.

The steady potential applied to the screen grid via the resistance Il is such that when the` signal shown at a of Figure 3 is applied the valve is made to alternate between the conditions illustrated by curves A and B of Figure 1, that is the output signals will be of the same and of reversed polarity with respect to the input signal during alternate half-cycles of the waveform of Figure 3a. With the waveform of Figure 3b applied to the signal grid, therefore, the output will be of the form shown in Figure 3c, alternate pulses being of opposite polarity.

There is an incidental effect which should be noted, namely that the application of the control voltages to the screen grid will alternately lower and raise the anode current somewhat and will produce at the anode of the valve a spurious signal whose wave form corresponds to wave form a of Figure 3. This effect is counteracted by also applying the control voltages to the signal grid of the valve I in a reduced amplitude which `is appropriately controlled to tend to produce Waveform a is that of the` an equal and opposite spurious signal at the anode so that compensation is obtained, i. e., the two spurious anode signals exactly cancel each other. The control voltages applied to the screen grid is also applied across resistances I2 and 8 in series, so that only a fraction thereof appears across resistance 8. This resistance is connected to the signal grid over resistance 'I and therefore this fraction of the signal is applied to the signal grid where it produces an opposing effect at the anode. More specically; as each positive-going square wave of the control voltage applied to the screen grid causes increased secondary emissions from the anode thus causing a decrease in anode current and a spurious increase in anode potential, a corresponding but smaller, positive square wave will be applied to the signal grid to appropriately increase anode current and decrease anode potential so that there is no appreciable over-all change in anode potential and vice-versa. By suitable choice of the values of resistances I2 and 8 the compensation may be made substantially complete. The resistance I3 is chosen such as to give an effective grid bias of suitable amount. It is not provided with a bypass condenser, as the negative feedback obtained by the omission of this component has a valuable straightening effect on the eiective characteristic of the valve.

It will be understood that the invention is not limited to the specic embodiment described with reference to Figures 2 and 3 which is capable of many modifications without departing from the scope of the invention.

What is claimed is:

1. Apparatus for controlling the polarity of electrical signals comprising a four element electron discharge device having a cathode, a signal grid, a screen grid and anode, means for applying direct potential to the screen grid and to the anode, means for applying the signals to the signal grid, means for applying control voltages to the screen grid for alternately reversing the direction of the -current to the anode so that the polarity of the signals appearing at the anode is also reversed, a voltage divider circuit, means for applying the control voltages to the divider circuit, means for applying a portion of the control voltages from the divider circuit to thev said signal grid, and means for taking the controlled electrical signals from the anode.

2. Apparatus for controlling the polarity of signals as in claim l, in which the control voltages comprise square waves.

3. An apparatus as in claim 1, and in which the potential of the screen grid alternately rises above and falls below the potential of the anode so that the secondary emission from the anode is alternately encouraged and prevented.

4. An apparatus as in claim 3, and in which the direct potentials of the screen grid and the anode have the same value and the control voltages comprise square waves of equal amplitude.

GEORGE E. G. GRAHAM.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,388,769 Shaffer Nov. 13, 1945 2,440,600 Crosby Apr. 27, 1948 2,464,594 Mahoney Mar. 15, 1949 

